1. Technical Field
Exemplary aspects of the present invention relate to a fixing device and an image forming apparatus, and more particularly, to a fixing device for fixing an image on a recording medium and an image forming apparatus incorporating the fixing device.
2. Description of the Background
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having two or more of copying, printing, scanning, facsimile, plotter, and other functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of a photoconductor; an optical writer emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to the image data; a development device supplies toner to the electrostatic latent image formed on the photoconductor to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the photoconductor onto a recording medium or is indirectly transferred from the photoconductor onto a recording medium via an intermediate transfer belt; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
The fixing device may employ an induction heater to heat the recording medium quickly. For example, the induction heater heats a fixing rotary body, such as a fixing belt and a fixing roller, pressingly contacted by a pressure roller to form a fixing nip therebetween. As the recording medium bearing the toner image is conveyed through the fixing nip, the fixing rotary body and the pressure roller apply heat and pressure to the recording medium, thus melting and fixing the toner image on the recording medium. Since the fixing rotary body incorporates a heat generation layer that generates heat by a magnetic flux generated by an exciting coil of the induction heater, the fixing rotary body is heated to a desired fixing temperature to fix the toner image on the recording medium quickly.
However, the heat generation layer is thin and therefore may cause temperature variation of the fixing rotary body in an axial direction thereof. For example, after a plurality of small recording media is conveyed over the fixing rotary body continuously, both lateral ends of the fixing rotary body in the axial direction thereof may overheat because the small recording media are not conveyed over both lateral ends of the fixing rotary body in the axial direction thereof and therefore do not draw heat therefrom. Accordingly, the temperature of the fixing rotary body varies in the axial direction thereof. Consequently, as a large recording medium is conveyed over the fixing rotary body immediately after conveyance of the small recording media, temperature variation of the fixing rotary body may vary gloss of a toner image on the large recording medium.
To address this problem, a self temperature control to offset a magnetic flux with a repulsive magnetic flux may be used. For example, a magnetic shunt alloy may be interposed between the heat generation layer and a metal plate serving as a degausser. When the temperature of the magnetic shunt alloy reaches a Curie temperature, a magnetic flux from the exciting coil penetrates the metal plate, allowing the metal plate to generate a repulsive magnetic flux that offsets the magnetic flux from the exciting coil.
In order to achieve the self temperature control, the exciting coil is situated in proximity to the magnetic shunt alloy. However, since the heat generation layer is disposed between the exciting coil and the magnetic shunt alloy, the degausser is situated in proximity to the heat generation layer. Accordingly, the degausser draws heat from the heated magnetic shunt alloy, elongating a warm-up time to warm up the heat generation layer to a target temperature.
To address this problem, two solutions are proposed. For example, as a first solution, as shown in JP-2013-003511-A, a part of the degausser that is requested to offset a decreased amount of the magnetic fluxes from the exciting coil is isolated from the heat generation layer with an increased interval therebetween, thus preventing the degausser from drawing heat from the heat generation layer. As a second solution, as shown in JP-2009-058829-A, the degausser rotates by 180 degrees with respect to the exciting coil, decreasing the repulsive magnetic fluxes generated by the degausser and thereby facilitating heat generation of the heat generation layer.
However, if the degausser is distanced from the exciting coil with an increased interval therebetween, the self temperature control of the degausser may degrade. Conversely, if the degausser is distanced from the exciting coil with a decreased interval therebetween, the degausser draws heat from the heat generation layer, degrading heat generation efficiency of the heat generation layer. For example, as the degausser is situated closer to the heat generation layer, the degausser is susceptible to magnetic fluxes leaked from the magnetic shunt alloy, which cause the degausser to generate repulsive magnetic fluxes that obstruct heat generation of the heat generation layer. Additionally, since the degausser is requested to generate an increased amount of repulsive magnetic fluxes to prevent temperature variation of the heat generation layer, it is necessary to locate the degausser close to the heat generation layer.
Accordingly, it is requested to locate the degausser at a position where the degausser enhances heat generation efficiency of the heat generation layer while generating a sufficient amount of repulsive magnetic fluxes to prevent temperature variation of the heat generation layer.